With respect to aromatic polycarboxylic acids, for example, 4,4'-biphenyldicarboxylic acid is useful as a raw material for high-performance polyesters, aramide resins, etc., and trimellitic acid is useful as a plasticizer for polyvinyl chloride and a raw material for polyamide-imide resins.
If such aromatic polycarboxylic acids can be obtained efficiently by one-stage oxidative reaction making use of, as a raw material, a benzil derivative having lower alkyl groups as side chains on the aromatic rings, this has a great significance in industrial technique.
By the way, in order to prepare an aromatic polycarboxylic acid by one-stage oxidative reaction from such a benzil derivative, it is necessary to oxidatively cleave an 1,2-diketone linkage (--COCO--) bonding two aromatic rings so as to convert two carbonyl groups thereof into carboxyl groups and at the same time, to oxidize the alkyl groups of the side chains into carboxyl groups.
The following various processes have been known heretofore as processes for oxidatively cleaving the 1,2-diketone linkage of benzil (namely, dibenzoyl):
(1) A process wherein benzil is reacted with 95% aqueous hydrogen peroxide into benzoic acid in the presence of 70% perchloric acid in acetic acid [J. Org. Chem., 16, 1785-7 (1951)].
(2) A process wherein benzil is reacted into benzoic acid in the presence of a complex of cuprous chloride and pyridine as a catalyst in methanol while introducing air [Bull. Chem. Soc. Jpn 32, 783-87 (1959)].
(3) A process wherein benzil is oxidized with sodium hypochlorite into benzoic acid under strong alkali
(4) A process wherein benzil is reacted with tert-butyl peroxide into benzoic acid in the presence of potassium hydroxide in chlorobenzene [Bull. Chem. Soc. Jpn 34, 102-4 (1961)].
(5) A process wherein benzil is reacted with potassium superoxide into benzoic acid in the presence of 18-crown-6-ether in benzene [J. Org. Chem., 41, 1077-8 (1976)].
(6) A process wherein benzil is reacted with hydroxylamine sulfonic acid in formic acid to concurrently conduct oximization and Beckmann rearrangement into benzoic acid [J. Org. Chem., 47, 2208-10 (1982)].
(7) A process wherein benzil is reacted with calcium hypochlorite into benzoic acid in a mixed solvent of acetonitrile and acetic acid [Tetrahedron Lett., 23, 3135-8 (1982)].
(8) A process wherein benzil is reacted with a cupric nitrate-pyridine-triethylamine complex into benzoic acid in methanol [J. Org. Chem., 49, 3498-3503 (1984)].
(9) A process wherein benzil is oxidized with dioxobis(trifluoroacetato)chromium into benzoic acid [Tetrahedron Lett., 27, 437-40 (1986)].
However, none of the above processes are suitable for a process wherein alkyl groups and an 1,2-diketone linkage (two carbonyl groups) of the benzil derivative having the alkyl groups as the side chains on the aromatic rings are concurrently oxidized into carboxyl groups to prepare an aromatic polycarboxylic acid by one-stage oxidative reaction.
Namely, by the processes (1) and (4), it is possible to oxidatively cleave said two carbonyl groups, but it is difficult to oxidize the alkyl groups bonded to the aromatic rings as side chains into carboxyl groups. With respect to the processes (2) and (8), the active species of the catalysts are Cu ions. These species have the ability to cleave said two carbonyl groups. However, it is also impossible to oxidize the alkyl groups bonded to the aromatic rings. By the process (3), an activated methyl or methylene group is oxidized into a carboxyl group. A side-chain alkyl group in toluic acid (namely, methylbenzoic acid) or the like is hard to be oxidized. Therefore, it is difficult to oxidize the side-chain alkyl groups at the same time that the 1,2-diketone linkage is oxidatively cleaved. The process (5) is difficult to practice industrially because it is not economical due to the use of an expensive crown ether. In the process (6), compounds formed by the cleavage of said two carbonyl groups are aromatic monocarboxylic acids or amides thereof. Another step is hence required to oxidize their side-chain alkyl groups. Also, by the process (7), only the oxidative cleavage of the 1,2-diketone linkage can be achieved, but the side-chain alkyl groups must be oxidized by another step. The process (9) involves a defect that the yield of the oxidative cleavage reaction itself to the 1,2-diketone linkage is low.
As described above, it is possible to oxidatively cleave the 1,2-diketone linkage of benzil or a benzil derivative into carboxylic groups by using any one of the above-described processes. However, it is difficult or substantially impossible to prepare an aromatic polycarboxylic acid by one-stage oxidative reaction from a benzil derivative having alkyl groups as side chains on the aromatic rings. These processes are hence accompanied by a defect that the process for preparing the aromatic polycarboxylic acid becomes at least two stages because at least one different step is required to oxidize each of the alkyl groups as the side chains.